curtain-raiser
n ~ (to sth)
(a) short piece performed before the main play 序幕(正剧演出前的短剧).
(b) thing that precedes a similar but larger or more important event 序幕, 前奏(重大事件的开端)
border incidents that were curtain-raisers to a full-scale war 引发一场全面战争的边境事件.